A communication channel is a general term used to describe a communications path, either physical or logical, between two systems. Radio waves are electromagnetic waves that can be used in wireless communication, and the radio frequency (RF) of these waves serves as physical communication channels. The RF spectrum has a finite range of frequencies, and thus a limited number of channels. In the United States, the Federal Communications Commission (FCC) decides how the spectrum is allocated, and what bands are used for what purpose. For example, television stations transmit generally high power RF signals in the VHF frequency range (54-88 MHz corresponding to channels 2-6 and 174-216 MHz corresponding to channels 7-13) and the UHF frequency range (470-806 MHz corresponding to channels 14-69). Currently the FCC is proposing to allow communications by unlicensed devices in these bands providing that they operate on a non-interfering basis.
Communication signals on adjacent channels can interfere with communications on the desired channel. This adjacent-channel interference can be caused by inadequate filtering, such as incomplete filtering of unwanted modulation products, improper tuning, or poor frequency control in the reference and/or the interfering channel. Adjacent-channel interference can increase with an increase in signal power in adjacent channels.